Twitter just announced its new two-factor authentication security system that can go a long way towards preventing unauthorized use of accounts.

Like ATM machines that require both a card and a pin number, most two-factor security systems require that you have something and know something. In Twitter's case you need to know your username and password and have your cell phone because, once you opt-in to this service, Twitter will send a code to your phone via SMS that you'll need to enter to log in.

But what if your cell phone is missing, dead or out of range?

Aside from the slight inconvenience of having to look at your phone and type in a 6-digit code, the biggest "risk" of using two-factor authentication is that you might not have access to your phone when you need to log in. If could be because you can't find it or because the battery is dead or because it's out of range -- all real possibilities.

Another problem is group use of Twitter accounts. It's not uncommon for colleagues to share a company Twitter account, but rarely do co-workers share a single cell phone.

Google Voice to the rescue

In both scenarios -- that missing phone or that group Twitter account -- the solution is Google Voice. Rather than register your actual cell phone number with Twitter, you register a Google Voice number (Google Voice is free and easy to set up). Google Voice lets you forward text (SMS) messages and phone calls to several phones (cell phones and landlines) so that an incoming text or call can be answered or viewed from any of the phones associated with your Google Voice account. For the individual Twitter user that means you can access that all important authentication code even if you don't have your cell phone, as long as you have access to the web and your Google account. Or you can route your Google messages to an alternate phone if yours is unavailable For groups it means that multiple people can get the same text message so they can all get the code when they need to log on.

So, now you have one less excuse for not turning on two-factor authentication.